Catalytic convertors are well known for the removal and/or conversion of the harmful components of exhaust gases. Catalytic convertors have a variety of constructions for this purpose. In one form the converter comprises the rigid skeletal monolithic substrate on which there is a catalytic coating. The monolith has a honeycomb-type structure which has a multiplicity of longitudinal channels, typically in parallel, to provide a catalytically coated body having a high surface area.
The rigid, monolithic substrate can fabricated from ceramics and other materials. Such materials and their construction are described, for example, in U.S. Pat. Nos. 3,331,787 and 3,565,830 each of which is incorporated herein by reference. Alternatively, the monoliths can be fabricated from metal foil.
The monolithic substrate and particularly the multiplicity of channels can be coated with a slurry of a catalytic and/or absorbent material.
One method of coating a prefabricated monolithic substrate is to pump the catalyst slurry into the respective channels and then subject the coated substrate to a drying operation. Such systems have been unsuccessful in providing a uniform coating thickness and a uniform coating profile wherein the catalyst coating is deposited over the same length of each of the channels.
It has been proposed to employ a vacuum to draw the catalyst slurry upwardly through the channels. For example, Peter D. Young, U.S. Pat. No. 4,384,014 discloses the creation of a vacuum over the monolithic substrate to remove air from the channels and then drawing the catalyst slurry upwardly through the channels. The vacuum is then broken and excess slurry is removed, preferably by gravity drainage.
James R. Reed, et al., U.S. Pat. No. 4,191,126, discloses the dipping of the monolithic substrate into a slurry and then utilizing subatmospheric pressure to purge the excess coating slurry from the surfaces of the support. The applied vacuum is intended to unplug the channels so that the slurry is drawn over the surfaces of each of the channels.
An improvement in these systems is disclosed in Thomas Shimrock, et al., U.S. Pat. No. 4,609,563, incorporated herein by reference. This system encompasses a method of vacuum coating ceramic substrate members with a slurry of refractory and/or catalyst metal components wherein precisely controlled, predetermined amounts of the slurry are metered for application to the ceramic monolithic substrate. The monolithic substrate is lowered into a vessel, also known as a dip pan, of preferably predetermined dimensions to a predetermined depth containing the precise amount of slurry which is to be coated onto the substrate. The slurry is then drawn up by a vacuum which is applied to the end of the substrate opposite to the end which is immersed in the bath. No draining or purging of excess coating slurry from the substrate is necessary nor is any pre-vacuum application step required to eliminate air.
A further improved method is disclosed in U.S. Ser. No. 08/962,363, filed Oct. 31, 1997, now U.S. Pat. No. 5,866,210, which is a continuation of U.S. Ser. No. 08/668,385 filed Jun. 21, 1996 and entitled, “METHOD FOR COATING A SUBSTRATE”. There is disclosed a vacuum infusion method for coating monolithic substrates in which each of the channels comprising the substrate is coated with the same thickness of the coating and is characterized by a uniform coating profile. The term “uniform coating profile” as used herein means that each channel of the substrate will be coated over the same length. In particular, the method is directed to a vacuum infusion method for coating a substrate having a plurality of channels with a coating media comprising:                a) partially immersing the substrate into a vessel containing a bath of the coating media, said vessel containing an amount of coating media sufficient to coat the substrate to a desired level without reducing the level of the coating media within the vessel to below the level of the immersed substrate;        b) applying a vacuum to the partially immersed substrate at an intensity and a time sufficient to draw the coating media upwardly from the bath into each of the channels to form a uniform coating profile therein; and        c) removing the substrate from the bath.        
Optionally, after the coating media is applied to the substrate and as the substrate is being removed from the bath, a vacuum continues to be applied to the substrate at an intensity equal to or greater than the intensity of the vacuum imposed on the partially immersed substrate.
The above referenced parent U.S. Pat. No. 5,866,210 which is a continuation of U.S. Ser. No. 08/668,385 now abandoned, discloses that a substrate may be inverted and coated from an opposite end producing two coatings having uniform coating profile. There is disclosed that if there is any overlap, it is much smaller than with prior art methods.
U.S. Pat. No. 5,953,832 discloses that after coating, the substrate or monolithic honeycomb can be rapidly and thoroughly dried without adversely affecting the coating profile. In particular, the disclosed method dries a monolithic substrate having a plurality of channels and a coating media thereon by removing the coated monolithic substrate from a bath containing the coating media while the coating media is in a wet condition. A vacuum is applied to the coated monolith substrate at an intensity in time sufficient to draw vapor out of the channels without substantially changing the coating profile within the channels. In a specific and preferred embodiment, the vacuum is imposed at one end of the substrate while gas at an elevated temperature is introduced into the opposite end of the substrate to facilitate drying.
Monolithic honeycombs containing different catalyst compositions in zones along the length of the honeycomb are known for use in catalytic combustion processes from references such as WO 92/09848. It is disclosed that graded catalyst structures can be made on ceramic and metal monoliths by a variety of processes. Monoliths can be partially dipped in washcoat and excess washcoat blown out of the channel. The process is repeated by dipping further into the washcoat sol. Alternatively, catalyst is disclosed to be applied to metal foil which is then rolled into a spiral structure. The washcoat is disclosed to be sprayed or painted onto the metal foil or applied by other known techniques such as by chemical vapor deposition, sputtering, etc.
It is also disclosed in WO 92/09848 that the catalyst can be applied as a mixture of active catalyst (such as palladium) and a high surface support (Al2O3, ZrO2, and SiO2, etc.). These are disclosed to be prepared by impregnating the palladium onto the high surface are oxide powder, calcining, then converting to a colloidal sol. In a second method, the high surface area washcoat may be applied first to the monolith or metal foil and fixed in place. Then the catalyst, e.g., palladium, may be applied by the same dipping or spraying procedure.
Three-way conversion catalysts (TWC) have utility in a number of fields including the treatment of exhaust from internal combustion engines, such as automobile and other gasoline-fueled engines. Emissions standards for unburned hydrocarbons, carbon monoxide and nitrogen oxides contaminants have been set by various governments and must be met, for example, by new automobiles. In order to meet such standards, catalytic converters containing a TWC catalyst are located in the exhaust gas line of internal combustion engines. The catalysts promote the oxidation by oxygen in the exhaust gas of the unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides to nitrogen.
Known TWC catalysts which exhibit good activity and long life comprise one or more platinum group metals (e.g., platinum or palladium, rhodium, ruthenium and iridium) located upon a high surface area, refractory oxide support, e.g., a high surface area alumina coating. The support is carried on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material.
U.S. Pat. No. 4,134,860 relates to the manufacture of catalyst structures. The catalyst composition can contain platinum group metals, base metals, rare earth metals and refractory, such as alumina support. The composition can be deposited on a relatively inert carrier such as a honeycomb.
In a moving vehicle, exhaust gas temperatures can reach 1000° C. or higher, and such elevated temperatures cause the activated alumina (or other) support material to undergo thermal degradation caused by a phase transition with accompanying volume shrinkage, especially in the presence of steam, whereby the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize alumina supports against such thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or strontia or rare earth metal oxides, such as ceria, lanthana and mixtures of two or more rare earth metal oxides. For example, see C. D. Keith, et al., U.S. Pat. No. 4,171,288. Reference is also made to a review of three-way catalysts in the Background of U.S. Ser. No. 08/962,283, filed Oct. 31, 1997 entitled, “CATALYST COMPOSITION”.
Preferred catalysts and catalyst structures which contain oxygen storage components are disclosed in WO 95/35152, WO 95/00235 and WO 96/17671 hereby incorporated by reference. These references disclose multiple layer catalysts. The discrete form and second coats of catalytic material, conventionally referred to as “washcoats”, can be coated onto a suitable carrier with, preferably, the first coat adhered to the carrier and the second coat overlying and adhering to the first coat. With this arrangement, the gas being contacted with the catalyst, e.g., being flowed through the passageways of the catalytic material-coated carrier, will first contact the second or top coat and pass therethrough in order to contact the underlying bottom or first coat. However, in an alternative configuration, the second coat need not overlie the first coat but may be provided on an upstream (as sensed in the direction of gas flow through the catalyst composition) portion of the carrier, with the first coat provided on a downstream portion of the carrier. Thus, to apply the washcoat in this configuration, an upstream longitudinal segment only of the carrier would be dipped into a slurry of the second coat catalytic material, and dried, and the undipped downstream longitudinal segment of the carrier would then be dipped into a slurry of the first coat catalytic material and dried.
There is a need to refine methods and articles to strategically locate catalyst on substrates.